In remarks made on the sidelines of last week's G-8 summit, Putin said Russia was prepared to cooperate with Ukraine if it did not pilfer Russian gas.

Tymoshenko said she did not "accept the humiliation of Ukraine and comments that bear no relation to reality."

Tymoshenko said no one in Ukraine has ever stolen a drop of gas and would never do so.

Accusations that Ukraine has been siphoning off gas from Soviet-era pipelines leading to Western Europe have plagued Russian-Ukrainian relations for years.

Last month, Russia's Gazprom monopoly announced the disappearance of 7,800 million cubic meters of gas and said it was unilaterally deducting the volume from fees it pays Ukraine for transit of supplies to European customers.